1. Field of the Invention
This invention relates to a method for preparing a liquid silicone rubber base useful as a source of silicone rubber and a method for preparing a liquid silicone rubber composition therefrom.
2. Prior Art
In the current silicone rubber market, liquid silicone rubber compositions enjoy an increasing share since they are smoothly flowing, readily applicable through injection molding machines and amenable to automatic processing. Among others, addition curing type liquid silicone rubber compositions based on alkenyl group-containing organopolysiloxane are on widespread use. Their preparation methods are generally designed so as to increase the manufacturing yield and accommodate for the manufacture of various types of products. Typically, a liquid silicone rubber base is once prepared as a common intermediate before a particular silicone rubber composition is prepared by blending the base with an appropriate curing agent and other ingredients.
The liquid silicone rubber base is generally prepared by blending an alkenyl-containing organopolysiloxane as a base component with a large amount of reinforcing silica filler. In JP-B 28737/1993, for example, a silicone rubber composition for injection-molding is prepared by blending 100 parts by weight of organopolysiloxane with up to 300 parts by weight of a filler as typified by finely divided silica.
However, when a large amount of reinforcing silica filler is blended to form a liquid silicone rubber base from which a liquid silicone rubber composition is prepared, no products having stable physical properties are obtained unless the silica filler is previously sufficiently surface treated or the silica filler is heat treated at relatively high temperature for a long time in the presence of a surface treating agent. JP-A 238228/1995, for example, discloses the addition of hexamethyldisilazane and heat treatment at room temperature for 1 hour and then at a temperature of 100.degree. to 250.degree. C. for at least 2 hours for restraining a viscosity increase with the lapse of time and imparting satisfactory shelf stability. Also JP-A 130344/1986 discloses heating/mixing treatment at 150.degree. C. for 2 to 3 hours. In either case, the processing time taken from the mixing of silicone oil as a base component with an inorganic filler to the completion of a base polymer is crucial. Heavy kneading or long-term heat treatment must be carried out to increase the rubber strength of the resulting liquid silicone rubber base polymer. A further long time is required in larger scale mass manufacturing plants.
Active research works have recently been made on the continuous process which generally relies on kneading operation using a continuous kneading machine including a single or twin screw extruder. In the continuous kneading machine, a certain limit is imposed on the ratio of the shaft length to the screw diameter since kneading operation is carried out while applying significant shear rates by high speed rotation. The residence time in the continuous kneading machine is naturally limited and generally as short as several tens of seconds to several minutes. Accordingly, a sufficient processing time is not ensured. As a result, the liquid silicone rubber base undergoes a viscosity increase and loses shelf stability. A liquid silicone rubber composition prepared from the base suffers from losses of basic rubber properties such as elongation and tensile strength and losses of fatigue endurance due to insufficient dispersion of the filler.
JP-B 28008/1992 discloses an attempt of heating a mixing section of a twin screw extrusion mixer at 200.degree. to 300.degree. C., thereby processing liquid silicone rubber base at a higher temperature to compensate for a shortage of processing time. Although this improves the rubber strength (tensile strength and elongation) of the resulting liquid silicone rubber composition, problems still remain with respect to viscosity and shelf stability. The liquid silicone rubber base and a liquid silicone rubber composition prepared from the base experience a change of flow with time. It is difficult to obtain a liquid silicone rubber base of stabilized quality.